[examples] Fix "nuxtjs" example by adding a yarn.lock file (#5882)

`@babel/preset-env` shipped a change that broke Nuxt.js usage of the
module, so here we add a `yarn.lock` file that pins the preset-env
version to 7.12.17.

See: https://github.com/nuxt/nuxt.js/issues/8882

examples/nuxtjs/.vercelignore

@@ -1,2 +1 @@
 README.md
-yarn.lock

examples/nuxtjs/package.json

@@ -13,6 +13,6 @@
     "nuxt": "^2.0.0"
   },
   "devDependencies": {
-    "nodemon": "^1.18.9"
+    "nodemon": "^2.0.7"
   }
 }